The present invention relates to a television receiver, and more particularly, to a combination VHF-UHF tuner in which a variable capacitance element is so arranged as to serve as a tuning element for electrically selecting a desired channel while a mixer employed in a VHF tuner serves as an intermediate frequency amplifier when a UHF channel is tuned in.
The abbreviation VHF and UHF used hereinabove and hereinbelow stands for very-high frequency and ultra-high frequency, respectively.
Conventionally, there have been proposed various types of the combination VHF-UHF tuner, one example is shown in FIG. 1. Referring to FIG. 1, there is shown a block diagram of the conventional tuner which includes a VHF tuner 1 having an input tuning circuit 2, a radio frequency amplifier, a primary interstage tuning circuit 3, a secondary interstage tuning circuit 4, a mixer 5, which are connected in series between a VHF input A and a tuner output B, and a local oscillator 6 connected to a junction Ja between the secondary interstage tuning circuit 4 and the mixer 5. The conventional tuner shown in FIG. 1 further includes a UHF tuner 7 connected between a UHF input C and a switch 8 which is in turn connected to the junction Ja.
According to the above described conventional tuner, particularly the one which is located at place where the VHF signal is strong, the input tuning circuit 2 and primary and secondary interstage tuning circuits 3 and 4 are normally set in a tuned condition with VHF high band signal, even when the UHF channel is tuned in, so that the VHF broadcasting signals carried in high channels are apt to be transmitted to the mixer 5. Therefore, in the case where the television receiver is set to receive UHF broadcasting signal, the VHF broadcasting signals carried in high channels may interfere with the UHF broadcasting signal.
Such interference may take place when the input tuning circuit 2 and primary and secondary interstage tuning circuits 3 and 4 are normally set in a tuned condition with VHF low band signals. In this case, the UHF broadcasting signal may be interfered by the VHF broadcasting signal of low channels.
Accordingly, a primary object of the present invention is to provide a tuning device which prevents the VHF signals from being transmitted to the mixer when the UHF signal is tuned in.
Another object of the present invention is to provide a tuning device of the above described type which is simple in construction and can readily be manufactured at low cost.
In accordance with a preferred embodiment of the invention, a combination VHF-UHF tuner comprises: a UHF tuner; a VHF tuner having an input tuning circuit, a primary interstage tuning circuit, a secondary interstage tuning circuit and a mixer connected in series between a VHF input and a tuner output and local oscillator; means connected to the UHF tuner for operating the same when UHF channel is tuned in; and means connected to the VHF tuner for making the input tuning circuit, primary interstage tuning circuit and secondary interstage tuning circuit operative to high band VHF signal when high channel VHF signal is tuned in, for making the input tuning circuit, primary interstage tuning circuit and secondary interstage tuning circuit operative to low band VHF signal when low channel VHF signal is tuned in, and for making at least one of the input tuning circuit, primary interstage tuning circuit and secondary interstage tuning circuit operative to high band VHF signal and the remaining operative to low band VHF signal when UHF channel is tuned in.